1. Field of the Invention
The present invention relates to an operational amplifier, and more particularly, to an operational amplifier having a common mode feedback circuit portion (hereinafter, referred to as CMFB) for self-adjusting a common mode voltage.
2. Description of the Related Art
As an example of operational amplifiers of this type, there is a known operational amplifier having a differential amplification portion that obtains a difference signal corresponding to a difference value between two input signals, a push-pull circuit that outputs a differential output signal according to the difference signal, and a CMFB circuit as described above.
In recent years, there is a proposal of a two-stage amplifier which is provided with a level-shift circuit in a stage preceding the push-pull circuit described above, in order to ensure that the amplifier operates under low power supply voltage (see FIG. 25, 1.2 of Non-Patent Literature 1 described below). In such a two-stage amplifier, a drive voltage that is obtained by shifting the level of the above-described difference signal by means of the level-shift circuit is supplied to gate terminals of a p-channel transistor and an n-channel transistor that together constitute the push-pull circuit (P-P Amp). The two-stage amplifier is provided with a common mode voltage generating circuit for generating a common mode voltage adjustment signal that indicates a difference value between a center value of difference output signals (OP, OM) of two routes and a predetermined common mode reference signal, and an n-channel transistors for adjusting a common mode voltage in the difference output signal (OP, OM) in accordance with the common mode voltage adjustment signal. The n-channel transistor provided for adjusting the common mode voltage is connected in parallel to the n-channel drive transistor in the push-pull circuit, and is configured to supply a current on the line of the difference output signal (OP, OM) in order to perform an adjustment to make the common mode voltage identical with the common mode reference voltage value.
Consequently, there has been a problem that the output current driving capability as the push-pull circuit is reduced by an amount corresponding to a current driving capability of the n-channel transistor that is provided for adjusting the common mode voltage.
Non-Patent Literature 1: Masato Yoshioka, Masahiro Kudo, Toshihiro Mori, Sanroku Tsukamoto, “A 0.8V 10 b 80 MS/s 6.5 mW Pipelined ADC with Regulated Overdrive Voltage Biasing,” ISSCC2007, Session 25 Nyquist ADC Techniques 25.1, pp. 452-453, Feb. 2007.